[libc][NFC] Reduce CMake configuration time

[gchatelet]

Summary

This patch reduces CMake configuration time drastically by removing a non-linear behavior.
Time to execute CMake configure step goes from 45s to 15s.

[sivachandra]

Thanks for looking into this!

libc/cmake/modules/LLVMLibCLibraryRules.cmake

 function(collect_object_file_deps target result)
   set(all_deps "")
   get_target_property(target_type ${target} "TARGET_TYPE")
   if(NOT target_type)
     return()
   endif()
 
   if(${target_type} STREQUAL ${OBJECT_LIBRARY_TARGET_TYPE})
     list(APPEND all_deps ${target})
     get_target_property(deps ${target} "DEPS")
     foreach(dep IN LISTS deps)
       collect_object_file_deps(${dep} dep_targets)
       list(APPEND all_deps ${dep_targets})
     endforeach(dep)
+    list(REMOVE_DUPLICATES all_deps)
     set(${result} ${all_deps} PARENT_SCOPE)
     return()
   endif()
 
   if(${target_type} STREQUAL ${ENTRYPOINT_OBJ_TARGET_TYPE})
     set(entrypoint_target ${target})
     get_target_property(is_alias ${entrypoint_target} "IS_ALIAS")
     if(is_alias)
       get_target_property(aliasee ${entrypoint_target} "DEPS")
       if(NOT aliasee)
         message(FATAL_ERROR
                 "Entrypoint alias ${entrypoint_target} does not have an aliasee.")
       endif()
       set(entrypoint_target ${aliasee})
     endif()
     list(APPEND all_deps ${entrypoint_target})
     get_target_property(deps ${target} "DEPS")
     foreach(dep IN LISTS deps)
       collect_object_file_deps(${dep} dep_targets)
       list(APPEND all_deps ${dep_targets})
     endforeach(dep)
+    list(REMOVE_DUPLICATES all_deps)
     set(${result} ${all_deps} PARENT_SCOPE)
     return()
   endif()
 
   if(${target_type} STREQUAL ${ENTRYPOINT_EXT_TARGET_TYPE})
     # It is not possible to recursively extract deps of external dependencies.
     # So, we just accumulate the direct dep and return.
     get_target_property(deps ${target} "DEPS")